The present invention relates to the field of aluminum electrolytic capacitors which include spacers comprising a porous layer of polymeric nanofibers.
Aluminum electrolytic capacitors are energy storage devices typically including an etched aluminum foil anode, an aluminum foil or film cathode, and a spacer interposed there between impregnated with a liquid electrolytic solution. The electrolytic solution provides ionic electrical conductivity from the cathode to an oxide layer formed on the aluminum anode which functions as a dielectric layer between the anode and the cathode. The multiple components are together rolled into a cylindrical body and encased, with the aid of suitable insulation, in an aluminum canister. Aluminum electrolytic capacitors can also be made with conductive polymer in place of liquid electrolytes. In these types of cells the spacers are used during cell winding and the rolled structures are then impregnated with the conductive polymer.
The spacer materials commonly used in aluminum electrolytic capacitors are papers, such as cellulosic papers. Aluminum electrolytic capacitors using these paper spacers desirably have a high level of protection against short-circuiting, but undesirably exhibit high ionic resistance and poor electrolyte absorption. Reduction of the density of the paper improves the ionic resistance and electrolyte absorption of the spacer at the expense of unacceptably increasing the tendency of the capacitor to short-circuit. In order to achieve the necessary balance between the electrolyte absorption of the spacer and the barrier to short-circuiting, at least one open porous layer of spacer paper is commonly combined with at least one dense layer of spacer paper. The resulting multiple layer structure provides adequate barrier properties and electrolyte absorption but the ionic resistance of the spacer is undesirably very high which leads to high ESR (equivalent series resistance) for the capacitor. Multiple layers of papers also result in thicker spacers, which in turn results in a device having lower capacitance.
Another problem with paper spacers for use in aluminum electrolytic capacitors is the nonuniform nature of the papers used, frequently containing particle impurities or void type defects. At higher voltages, these nonuniformities can lead to direct current leakage or even failure of the capacitors. Thus more than one layer of paper, typically 2 to 6 layers, are used to mask these nonuniformities and generally achieve good electrical properties. Use of more than one layer of paper increases the capacitor size or reduces the capacitance and also presents problems in evenly rolling the electrodes and paper spacers. Use of multiple layers can also lead to higher ESR because of poor contact between different layers of the spacers. All the above issues are undesirable and can lead to performance and manufacturing efficiency loss.
Polymer spacers, in the form of microporous film or fabric, have also been used in aluminum electrolytic capacitors. An example of a capacitor spacer formed from polytetrafluoroethylene microporous film is disclosed in U.S. Pat. No. 3,661,645 to Strier et al. U.S. Pat. No. 5,415,959 to Pyszeczek et al. describes the use of woven fabrics of synthetic halogenated polymers as capacitor spacers. The use of “hybrid” spacers comprising polymer (porous film made from polypropylene or polyester) and paper material is disclosed in U.S. Pat. No. 4,480,290 to Constanti et al. A major problem with the use of microporous film spacers in aluminum electrolytic capacitors is that the ionic resistance is usually unacceptably high. It is believed that sufficient electrolyte is not allowed to contact the electrodes as a result of the inherent limited porosity of microporous film spacers. Nonwoven fabrics made with large fiber size offer low ionic resistance but they are usually very thick and non-uniform leading to poor barrier properties.
There is a need for aluminum electrolytic capacitors having increased life and improved performance, and for improved aluminum electrolytic capacitor spacers having a desirable balance of thickness, electrolyte absorption, ionic resistance and barrier to short-circuiting.